This invention relates to electrical cables, and more particularly to strain relief connections at the termination of cables.
Flexible electrical cables are used to provide connections between fixed instruments and movable remote devices. For instance, an ultrasound instrument has a transducer unit connected at the free end of a flexible cable. Such cables require flexibility for convenient and comfortable use, and often have a multitude of fine conductors in the cable bundle. In other applications, cables extend between connector housings at each end. Such conductors may be relatively fragile, and subject to damage. Even minor damage can cause wire performance characteristics to deviate from a required range.
In such applications, cables must be able to withstand expected incidents of misuse or accidental tension on the cable. Without adequate provision for strain relief to handle such tension, wires in the cable may transmit the tension to their connections to circuitry in the instrument or remote unit, causing the connections to fail. To avoid this, existing strain relief mechanisms grip the entire cable so that all conductors and a jacket bear the strain in concert, and so that the strain is borne at a cable neck portion away from delicate connections. However, this generates crushing of the wires, which can cause damage. Even without damage, pressure can reduce intended spacing between wires, leading to performance problems due to unintended crosstalk characteristics.
To avoid this, some existing strain relief mechanisms employ tapered cone and cup mechanisms that crimp the cable jacket between the conical outer surface of the cone, and the conical interior bore surface of the cup. However, these systems have at least one of two disadvantages. The first disadvantage is that such systems often have split cone rings that generate permanent compression of the cable bundle, potentially affecting performance, and also reducing cable flexibility as wires are fixed at the mechanism, and unable to slide freely with respect to each other. The second disadvantage is that such tapered cone mechanisms use threaded clamping nuts that are tightened to generate axial force between the cone and cup to pinch the cable jacket. This approach is susceptible to over and under tightening. With under tightening, the clamp may fail when the tension exceeds a threshold; with over tightening, the mechanism or cable components may be damaged.
The present invention overcomes the limitations of the prior art by providing a cable assembly including a cable having a core surrounded by a sheath. A cone element has an interior bore receiving the cable core, and has a tapered exterior surface. At least a portion of the cone is received within the cable sheath. A cup has a tapered bore receiving the sheath and the core. A portion of the sheath is captured between the exterior of the cone and the interior of the cup bore. Tension applied to the cable with respect to cup generates compression of the sheath between the cone and cup, and tension is thus transmitted by the sheath. Application of a light radial force is applied to the core only when tension is applied to the sheath. Under this condition the sheath tends to both elongate and reduce in diameter until the core of the cable limits further diameter reduction. The sheath may include a strength member such as a braided metal wrap, and an elastic jacket. As the wrap bears the tension it elongates and reduces in diameter, as noted above, until the core of the cable limits further diameter reduction. When the core limits further reduction in diameter the braided metal wrap acts to prevent further elongation and to bear the load. When the tension is released the elastic jacket causes the strength member to retract to its original length, which in turn returns it to its original diameter. This then removes any radial force from the core.